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Mutation • The ultimate source of genetic variation; however it usually occurs at low enough frequency to have only minor effects on gene frequency over short evolutionary time spans • Exception - non-coding regions of DNA can evolve very rapidly due to a combination of relaxed selection and high mutation rates Genetic Drift Alteration of gene frequencies due to chance (stochastic) effects • Most important in small populations • An agent that tends to reduce genetic variation as the result of extinction of alleles • Generally does not produce a fit between organism and the environment; can, in fact, result in non-adaptive or maladaptive changes 1 Genetic Drift: Founder Effects • Hawaiian Drosophila: 400 species derived from one or few colonist species • Some evolutionary biologists argue that founder effects and genetic bottlenecks are of major importance in major evolutionary changes Non-Random Mating Changes genotype frequencies but not gene frequencies • Assortative mating: Like phenotypes mate preferentially with one another (e.g., tall with tall); increases homozygosity at some, but not all, loci • Disassortative mating: dissimilar phenotypes mate preferentially (e.g., short with tall); increases heterozygosity; might be important at MHC loci • Inbreeding: mating between close relatives which produces a deficit of heterozygotes; inbreeding exposes lethal recessive alleles and usually depresses fitness 2 • http://www.pbs.org/wgbh/evolution/librar y/01/6/l_016_08.html Migration Movement followed by breeding in a new location • Migration can reduce local adaptation, particularly in species with broad geographic ranges – – – – e.g., the white-tailed deer mountain lion mussels: salinity tolerance heavy metal tolerance in plants in England 3 Selection • Natural selection occurs when attributes of the phenotype, e.g., size, color, and enzymatic activity, vary with in a consistent way (covary) with Darwinian fitness (the product of survival and reproduction) — Directional — Stabilizing — Disruptive Sexual selection is the differential reproductive success resulting from competition between members of one sex, usually males, to achieve matings and/or fertilizations; can result in the evolution of exaggerated male traits which may be detrimental to survival Natural Selection • The superior survival and/or reproduction of some phenotypic variants compared to others under the environmental conditions that prevail at the moment • Natural selection occurs when attributes of the phenotype, e.g., size, color, and enzymatic activity, vary with in a consistent way (=covary) with Darwinian fitness (the product of survival and reproduction) 4 What’s Different about NS? • The other forces of evolution, i.e., genetic drift, inbreeding and gene flow, tend to act at the same rate on all loci • Natural selection: causes allele frequency changes that proceed independently at different loci —Different characteristics of a population evolve at different rates (mosaic evolution) • NS produces adaptations: i.e., traits that increase the ability of an individual to survive or reproduce compared to individuals lacking the trait Natural Selection & Evolution • Natural selection occurs when different phenotypes vary in average reproductive success • However, natural selection is different from evolution by natural selection • Natural selection can have no evolutionary impact if unless phenotypes differ in genotype (variation must have a genetic basis) 5 Types of Natural Selection Directional: • One extreme phenotype is fittest (e.g., small or large individuals) • Acts to drive favored allele to fixation Stabilizing: • The intermediate (average) phenotype is fittest; probably the most common form of selection • e.g., heterozygote advantage Disruptive: • Two or more phenotypes are fitter than the intermediates between them • e.g., heterozygote disadvantage Directional Selection • • Many examples of strong directional selection associated with human-caused environmental change, e.g., industrial melanism; insecticide resistance in insects, heavy metal tolerance in plants, or with Components of fitness, such as survival after food depletion in Cliff Swallows 6 Directional selection: evolution of insecticide resistance Directional Selection • Many good examples of strong directional selection associated with human-generated environmental change, e.g., industrial melanism; insecticide resistance in insects; heavy metal tolerance in plants 7 Stabilizing Selection: Birth Weight • The most common form of selection • Results from tradeoffs: an inescapable compromise between one trait and another Disruptive Selection • Bimodal distribution of bill size in Blackbellied seedcracker • Birds can either specialize on hard seeds (wide bills) or soft seeds (narrow bills) 8 Classic Example of Disruptive Selection: Gamete Size in Animals Negative Frequency Dependent Selection • Fitness of a genotype increases when it becomes rare • Fitness of a genotype decreases when it becomes common 9 Negative Frequency Dependent Selection: Sex Ratio 10